Is the United Kingdom subject to the EASA's authority or the CAA's (Civil Aviation Authority) authority?
EASA: this is a European Union body, so yes, the UK is a member state.
CAA: There are many Civil Aviation Authorities worldwide, basically each is a national entity within the corresponding state. So, specifically for the UK, yes, there exists a body known as CAA that is a state entity, formally independent but in practice dependent on the UK Government (Dept. of Transport). Their web page is: http://www.caa.co.uk/
Due to the complex configuration of EU law, both EASA and the UK CAA can have their word to say on operations within UK territory. Though, as CAA say themselves:
The CAA works closely with the European Aviation Safety Agency (EASA) to promote the highest common standards of safety and environmental protection in civil aviation.